1. Technical Field
The present invention relates to a plurality of photosensitive bodies, a plurality of developing devices that use liquid developer containing non-volatile solvent as carrier to develop electrostatic latent images formed on the respective photosensitive bodies, a transfer body that sequentially transfers toner images developed by transfer units corresponding to the plural photosensitive bodies and stacks the toner images, a liquid developer collecting system that controls concentration of liquid developer collected from the developing devices and reuses the liquid developer, and also relates to an image forming apparatus including these components.
2. Related Art
Various types of wet-type image forming apparatus that develop a latent image using high-viscosity liquid developer containing toner formed by solid components and dispersed in liquid solvent to visualize an electrostatic latent image have been proposed. The liquid developer used in a typical wet-type image forming apparatus contains solid components (toner particles) suspended in electricity-insulation organic solvent (carrier) such as silicon oil, mineral oil and edible oil. The particle diameter of the toner particles may be as small as about 1 μm. By using such fine toner particles, the wet-type image forming apparatus can produce higher quality images than those produced by a dry-type image forming apparatus, which uses powder toner particles typically having particle diameters of about 7 μm.
An image forming apparatus of the type using liquid developer has been proposed in which liquid developer collected from the developing device or photosensitive body is reused. According to one such image forming apparatus in the related art, a thin layer of liquid developer having a thickness of 1 to 50 μm is applied to a developing roller, and sent to a developing nip. Liquid developer that has passed the developing nip and remains on the developing roller is scraped by a blade and stored in a collection section. Then, solid particles of the collected liquid developer are shifted onto the photosensitive body, where the liquid developer is diluted. The carrier rate of liquid developer collected from the photosensitive body is high, and thus its solid concentration is lower than that of liquid developer collected from the developing device.
The diluted liquid developer is sent to a concentration control unit by using a pump or the like. Then, the diluted developer is mixed with high-concentration liquid toner supplied thereto to adjust the concentration of the developer to a target solid concentration. The liquid developer having the target solid concentration is again sent to the developing device and reused (see JP-A-2002-6637).
However, the proportion of the solid particles in the collected liquid developer is not constant. Typically, the consumption amount of the solid particles varies according to image data. For example, when the image data corresponds to full-tone, many solid particles contained in the liquid developer collected from the developing roller after development by using a developing roller cleaning blade are shifted to the photosensitive body and consumed. Thus, the collected liquid developer has a lower solid concentration. When the image data corresponds to half-tone, a small amount of the solid particles are shifted to the photosensitive body, and the solid concentration of the collected liquid developer thus changes little. Thus, the solid concentration needs to be adjusted to a target concentration by using a concentration control device when the solid concentration is equal to or lower than an allowable predetermined value in case of reuse of the collected liquid developer whose solid concentration varies. According to a color image forming apparatus, the concentration control device is provided for each color to prevent color mixture. In order to meet demand for size reduction of the image forming apparatus, the capacity of the concentration control device provided for each color needs to be reduced.
In order to adjust a low concentration of a collected liquid developer to a predetermined concentration using a concentration control device having a small capacity, a high-concentration new toner is supplied to the concentration control device from a toner tank. The concentration of the new toner may be, for example, about 35%. Thus, for example, when the concentration of the collected liquid developer is 17% under the condition of a predetermined concentration set at 20%, a predetermined amount of the new toner having the concentration of 35% needs to be supplied to adjust to the predetermined concentration by the concentration control device. In this case, the concentration cannot be efficiently adjusted when the concentration control device does not have sufficient vacant capacity.